lib/roken/ct.c

   1 /*
   2  * Copyright (c) 2009 Kungliga Tekniska Högskolan
   3  * (Royal Institute of Technology, Stockholm, Sweden).
   4  * All rights reserved.
   5  *
   6  * Redistribution and use in source and binary forms, with or without
   7  * modification, are permitted provided that the following conditions
   8  * are met:
   9  *
  10  * 1. Redistributions of source code must retain the above copyright
  11  *    notice, this list of conditions and the following disclaimer.
  12  *
  13  * 2. Redistributions in binary form must reproduce the above copyright
  14  *    notice, this list of conditions and the following disclaimer in the
  15  *    documentation and/or other materials provided with the distribution.
  16  *
  17  * 3. Neither the name of the Institute nor the names of its contributors
  18  *    may be used to endorse or promote products derived from this software
  19  *    without specific prior written permission.
  20  *
  21  * THIS SOFTWARE IS PROVIDED BY THE INSTITUTE AND CONTRIBUTORS ``AS IS'' AND
  22  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  23  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  24  * ARE DISCLAIMED.  IN NO EVENT SHALL THE INSTITUTE OR CONTRIBUTORS BE LIABLE
  25  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  26  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  27  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  28  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  29  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  30  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  31  * SUCH DAMAGE.
  32  */
  33
  34 #include <config.h>
  35 #include "roken.h"
  36
  37 /**
  38  * Constant time compare to memory regions. The reason for making it
  39  * constant time is to make sure that timeing information leak from
  40  * where in the function the diffrence is.
  41  *
  42  * ct_memcmp() can't be used to order memory regions like memcmp(),
  43  * for example, use ct_memcmp() with qsort().
  44  *
  45  * We use volatile to avoid optimizations where the compiler and/or
  46  * linker turn this ct_memcmp() into a plain memcmp().  The pointers
  47  * themselves are also marked volatile (not just the memory pointed at)
  48  * because in some GCC versions there is a bug which can be worked
  49  * around by doing this.
  50  *
  51  * @param p1 memory region 1 to compare
  52  * @param p2 memory region 2 to compare
  53  * @param len length of memory
  54  *
  55  * @return 0 when the memory regions are equal, non zero if not
  56  *
  57  * @ingroup roken
  58  */
  59
  60 int
  61 ct_memcmp(const volatile void * volatile p1,
  62           const volatile void * volatile p2,
  63           size_t len)
  64 {
  65     /*
  66      * There's no need for s1 and s2 to be volatile; only p1 and p2 have
  67      * to be in order to work around GCC bugs.
  68      *
  69      * However, s1 and s2 do have to point to volatile, as we don't know
  70      * if the object was originally defined as volatile, and if it was
  71      * then we'd get undefined behavior here if s1/s2 were declared to
  72      * point to non-volatile memory.
  73      */
  74     const volatile unsigned char *s1 = p1;
  75     const volatile unsigned char *s2 = p2;
  76     size_t i;
  77     int r = 0;
  78
  79     for (i = 0; i < len; i++)
  80         r |= (s1[i] ^ s2[i]);
  81     return !!r;
  82 }